I. Field of the Invention
This invention generally relates to the field of electrodeposition, and, specifically, to the electrodeposition of tin-lead alloys from aqueous acid baths.
II. Description of the Prior Art
Tin-lead alloys have been successfully deposited from acid fluoborate (HBF.sub.4) baths. However, in order to reduce the tendency of the fluoborate baths to form non-adherent, treed and/or dark deposits, certain additives must be incorporated into these baths. Additives which have long been known and used for this purpose include such materials as glue, gelatin, hydroquinone, peptone and so forth.
It is of particular interest to electrodeposit tin-lead alloys of 60% tin and 40% lead which are bright. It is also of interest to deposit tin-lead alloys having low concentration of lead, as for example, from about 1 to 2% lead.
N. Dohi in "Metal Finishing" Vol 64 p. 62-3 (October 1966) discloses a bright tin-lead HBF.sub.4 bath employing an ethylene oxide condensation product as a wetting agent, formaldehyde (20 ml/liter) and an acetaldehyde/ortho-toluidine reaction product (Schiff base).
French Pat. No. 2,096,936 discloses a bright tin-lead (60-40) acid fluoborate bath containing as an organic additive a substituted phenol such as 2,4,6-trimethyl phenol, an aldehyde such as formaldehyde in the amount of from 30 to 50 gm of a 38% solution as a reducing agent and an ethoxylated high molecular weight amine or alkylamine as a wetting agent.
U.S. Pat. No. 3,661,730 discloses a bright tin-lead bath employing formaldehyde (10-30 ml/liter), an ethylene oxide condensation product as a wetting agent and a Schiff base (a reaction product of an aldehyde and an amine) or orthotoluidine.
U.S. Pat. No. 2,446,716 discloses a bath employing nicotine and a wetting agent such as Duponol 80 (Dupont's sodium salt of technical octyl alcohol sulfate). The combination of nicotine, alkylene oxide condensation products and an aromatic aldehyde, however, do not produce acceptably bright tin-lead deposits.
None of the aforesaid baths have attained sufficient commercial acceptance due to several major disadvantages attributable to them.
The concentrations of formaldehyde indicated are comparatively high and result in highly objectionable and irritating odors.
The Schiff base reaction products of the organoamines and aromatic aldehydes are unstable. Upon standing idle or after a short period of electrolysis, dull, low-current density areas will result. Excess use of corrective brighteners must be made in order to overcome the low current density problem. This leads to difficulty in the adjustment of the bath and also makes the bath expensive to operate.
The aforedescribed baths with continued operation or with periods of idleness have only a narrow bright plating range. If the low current density areas are satisfactory, the high current density areas are dull, the reverse also being true.